Is it permissible for me to pray alone to attain khushu' (devotional humility), even if there is a congregation praying or about to begin praying?

The sense of devotion (khushū‘) you feel when praying alone, without congregational prayer, is a ploy from Satan to divert you from the obligation of congregational prayer. Indeed, congregational prayer is obligatory. Allah has encouraged it in His Book and commanded it even during wartime. The Prophet (peace be upon him) warned against abandoning it and explained that Satan gains control over those who forsake it. Furthermore, among the benefits of congregational prayer are mutual acquaintance and cooperation in righteousness. Abandoning it may lead to abandoning prayer entirely, which is disbelief (kufr). Allah does not mandate a ruling if good lies elsewhere. Therefore, one must perform congregational prayer and achieve devotion within it, while taking the means to attain devotion.